1. Field of the Invention
The present invention relates to a water electrolysis apparatus for producing hydrogen through electrolysis of water, the apparatus including an electrolyte membrane, first and second current collectors disposed on the respective opposite sides of the electrolyte membrane, first and second separators stacked on the current collectors, a first flow field defined by stacking the first current collector and the first separator, and a second flow field defined by stacking the second current collector and the second separator.
2. Description of the Related Art
Solid polymer electrolyte fuel cells generate DC electric energy when anodes thereof are supplied with a fuel gas, i.e., a gas mainly composed of hydrogen, e.g., a hydrogen gas, and cathodes thereof are supplied with an oxygen-containing gas, a gas mainly composed of oxygen, e.g., air.
Generally, water electrolysis apparatus are used to generate a hydrogen gas for use as a fuel gas for such solid polymer electrolyte fuel cells. The water electrolysis apparatus employ a solid polymer electrolyte membrane (ion exchange membrane) for decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are disposed on the respective sides of the solid polymer electrolyte membrane, making up a membrane electrode assembly. Current collectors are disposed on the respective opposite sides of the membrane electrode assembly, making up a unit. The unit is essentially similar in structure to the fuel cells described above.
A plurality of such units are stacked, and a voltage is applied across the stack while water is supplied to the current collectors on the anode side. On the anodes of the membrane electrode assemblies, the water is decomposed to produce hydrogen ions (protons). The hydrogen ions move through the solid polymer electrolyte membranes to the cathodes, where the hydrogen ions combine with electrons to generate hydrogen. On the anodes, oxygen generated together with hydrogen is discharged with excess water from the units.
Such a water electrolysis apparatus generates hydrogen under a high pressure of several tens MPa. There is known a hydrogen supply apparatus as disclosed in Japanese Laid-Open Patent Publication No. 2004-002914, for example. As shown in FIG. 13 of the accompanying drawings, the disclosed hydrogen supply apparatus includes a number of unit cells each comprising an assembly which has an anode current collector 2, a cathode current collector 3, and an electrode assembly membrane 1 disposed between the collectors 2 and 3, and a pair of bipolar plates 4 sandwiching the assembly therebetween.
A flow field 5a for supplying water therethrough is defined between one of the bipolar plates 4 and the anode current collector 2, and a flow field 5b for passing generated hydrogen therethrough is defined between the other bipolar plate 4 and the cathode current collector 3. Each of the bipolar plates 4 has first seal grooves 7a, 7b defined in a peripheral edge portion thereof and accommodating first o-rings 6a respectively therein and second seal grooves 7c, 7d defined in a peripheral edge portion thereof and accommodating second o-rings 6b respectively therein.
In the above Japanese Laid-Open Patent Publication No. 2004-002914, the flow field 5b serves as a high-pressure hydrogen generating chamber for generating high-pressure hydrogen. The second seal groove 7d, which is held in fluid communication with the flow field 5b, is filled with the high-pressure hydrogen, developing a high pressure therein. As a result, the hydrogen slightly leaks through the second o-rings 6b, and then stagnates between the second seal groove 7d and the first seal groove 7b. 
Thus, the stagnating high-pressure hydrogen causes reaction force between the second seal groove 7d and the first seal groove 7b, and the reaction force tends to disrupt a balance of pressing forces externally-applied to the entire hydrogen supply apparatus in the stacking direction. Consequently, the electrode assembly membrane 1 can not be maintained in an appropriate pressed condition, and then the electrolysis voltage increases, resulting in decrease of the electrolysis performance.